The Mandarin word "gĕilì" (给力) literally means "to give more power" or "to give more force". Thanks to the ever-increasing spread of English in China, this has given rise to the wonderful portmanteau "ungelivable".
A mixture of "gĕilì" and "unbelievable", the slang word originating on the internet has come to mean "not strong enough", and is an interesting example of how two very different languages can sometimes fit together to create a fairly seamless compound.
While it's not going to be making its way into the English or Chinese dictionaries any time soon, "ungelivable" also shows how powerful the internet is as a tool to spread ideas. The word spread so quickly, especially among the younger generation, that knowledge of it has rapidly become commonplace - even if usage has not.